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THE AMERICAN NATURALIST [Vol. L 



the requirement after 9 years of age, hence by selection, 

 only the best animals were retained, thns drawing the 

 curve down almost to a straight line. The tendency of 

 the curve, however, is to recede, showing that the animals 

 of these ages should not be weighted as heavily as younger 

 animals. A study of a number of representatives of the 

 whole breed would be necessary to determine this point. 



One of the most striking points shown by these data 

 and one which substantiates the opinion of practical 

 breeders of Holsteins, also brought out in the practical 

 investigations of Eccles, 1 is the difference in production 

 and capacity between 2- and 2^-year-old and 3-year-old 

 cows. The difference in the means of the production 

 between 2 and 2^ years was 18 pounds only, while be- 

 tween 2\ and 3 years it was 46 pounds, or a total of 64 

 pounds between the 2- and 3-year classes. Between the 

 3- and 4-year classes the difference is almost as great, 

 being 56 pounds, but the deviation of the latter class is 

 not quite as great as the former. This seems to indicate 

 that the 3-year animal is still at a disadvantage by reason 

 of its immaturity in growth and body development. That 

 the average deviation of 2|-year class was 43 pounds 

 while the 3-year class deviated 56 pounds is significant 

 also and leads to the conclusion that at 1\ years of age 

 the Holstein is still growing, and this, combined with the 

 great strain of milk production, limits the capacity of 

 the class. 



It may be said by some that few 3- and 4-year-old 

 animals are tested for advanced registry in comparison 

 to two year olds and aged animals, and in consequence 

 of this, only the best of the class make the requirements. 

 This is not borne out by the data, the number in the 3 

 year-old class being second largest of all animals. 



Curves of Fat Production 

 A study of the curves based upon the actual fat pro- 

 duction of this population (Fig. 2) brings out a number 



» Bui. No. 135, Missouri Agricultural Exp. Station. 



